To infect lung airways, viruses must penetrate mucus. However, little is known about how or the efficiency with which respiratory viruses can diffuse across airway mucus (AM). In this proposal, using human AM collected from healthy volunteers, we first seek to characterize the mobility of four common respiratory viruses in fresh AM ex vivo, and identify which virus readily penetrates AM and which virus is hindered or trapped. We have developed different sized muco-inert synthetic nanoprobes that reveal the mesh spacing (pore size) and nanoscale viscoelasticity of fresh human mucus secretions. Thus, for viruses that are slowed in AM, we can determine whether the limited mobility is caused by steric occlusion and/or by adhesion to mucus constituents. Recently, we discovered that the mesh spacing in human cervicovaginal mucus is much larger than mammalian viruses, consistent with our earlier observations that HIV, HPV and Norwalk virus all readily diffuse through the same mucus secretions. Hypothesis #1: the mesh spacings in AM are larger than most respiratory viruses, and viruses will readily penetrate AM if they are not slowed by adhesive interactions. Thus, one approach to block pulmonary infections is to adhesively trap viruses in mucus. Numerous studies demonstrate that antibodies (Ab) applied topically to mucosal surfaces, including the lung airways, can provide robust protection against infections, some even at sub-neutralizing concentrations. The immune system secretes more Ab into mucus than blood or lymph, but the mechanisms by which Ab in mucus protect against infections remain poorly studied. Hypothesis #2: Array of virus-bound Ab can form multiple lowaffinity adhesive crosslinks between the virus and the mucus gel. A sufficient number of these low-affinity crosslinks, possibly at sub-neutralizing concentrations, permanently trap viruses in the mucus gel. Trapping reduces flux of virions that can reach target cells, enables rapid elimination via mucociliary clearance, and facilitates viral degradation and inactivation by other protective mechanisms. Our pilot observations indicate that remarkably low concentrations of specific IgG1 can trap HSV and virus-like particles that otherwise rapidly penetrate mucus gels. Aim 1: Measure the transport rates of common respiratory viruses, including adenovirus, influenza, and rhinoviruses, in human AM ex vivo obtained from healthy subjects. We will also characterize the microstructure of AM to determine whether trapped viruses that are immobilized by steric or adhesive interactions. Aim 2: Determine whether virus- specific Ab (IgG, IgA, sIgA, IgM) secreted into native AM may facilitate adhesive trapping of viruses in AM, and investigate whether addition of exogenous virusspecific IgG trap viruses that otherwise rapidly penetrate AM. Together, Aim 1 and 2 will provide a quantitative description of how respiratory viruses may penetrate AM, and how Ab may protect the airways by trapping viruses in AM. The results will likely aid in developing new vaccines or engineering Ab that block infections in the lung airways by trapping virions in AM.